Despite that Test-Driven Development (TDD) appears to be a software testing practice, many developers affirm that the practice influence on class design. This study aimed to better understand the effects of TDD and how the practice influences developers during class design on object-oriented systems. We conducted an essentially qualitative exploratory study in which participants were invited to implement some exercises using TDD and, based on the data gathered, we gathered details of how the practice influenced design decisions from the participants using interviews. We noticed that the practice of TDD drives developers during class design by means of constant feedback about its quality. This study also named and catalogued feedback patterns perceived by the developers.